Securities regulators fined Knight Capital Americas LLC $12 million for the trading malfunction that roiled the U.S. stock market in August 2012, saying the firm ignored dozens of error messages before its computers bombarded exchanges with millions of unintended orders.

Thomas Joyce, who built Knight Capital Group Inc. into one of the biggest U.S. market making firms only to see it driven to the brink of bankruptcy and then into a suitor’s arms, is leaving the company.

Knight Capital Group Inc., one of the largest U.S. market makers, shut down trading of equities today after backup power failed at its headquarters in Jersey City, New Jersey, amid a blackout following Hurricane Sandy.

Erroneous trades that sent Kraft Foods Group Inc. up as much as 29 percent in the first minute of trading were canceled by exchanges, the latest incident to fuel scrutiny of the electronic infrastructure of U.S. markets.

Eliminating all trading errors is impossible and the way to address malfunctions that have plagued equity markets this year is to improve testing and oversight, industry executives said at a meeting in Washington.

Jefferies Group Inc., the investment bank that helped rescue Knight Capital Group Inc., dropped the most in eight months in New York trading after reporting earnings that fell short of analysts’ estimates excluding gains from the deal.

A trading loss that almost sent Knight Capital Group Inc. into bankruptcy may spur regulatory changes to protect against future errors by “knuckleheads,” Knight Chief Executive Officer Thomas Joyce said.